Reel-to-reel plating of conductive grids for flexible thin film solar cells

ABSTRACT

The present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells. In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application No: 61/169673 filed Apr. 15, 2009 entitled “Reel to Reel Plating of Conductive Grids for Flexible Thin Film Solar Cells”, the entirety of which is incorporated herein by reference.

BACKGROUND

1. Field of the Inventions

The present inventions generally relate to solar cell fabrication and, more particularly, to fabrication of flexible thin film solar cells.

2. Description of the Related Art

Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1−x)Ga_(x) (S_(y)Se_(1−y))_(k), where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. It should be noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)₂ means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.

The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown in FIG. 1A. A photovoltaic cell 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)₂, is grown over a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the conductive layer 13 form a base 20 on which the absorber film 12 is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of FIG. 1A. If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent conductive layer 14 such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film 12. Radiation 15 enters the device through the transparent conductive layer 14. As shown in FIG. 1B in top view, metallic grids 30 may also be deposited over top surface 16 of the transparent layer 14 to reduce the effective series resistance of the device. The top surface 16 forms the illuminated surface of the solar cell 10. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent conductive layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1A is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)₂ absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side.

If the substrate 11 of the CIGS(S) type cell shown in FIG. 1A is a metallic foil, then under illumination, a positive voltage develops on the substrate 11 with respect to the transparent layer 14. In other words, an electrical wire (not shown) that may be attached to the substrate 11 would constitute the (+) terminal of the solar cell 10 and a lead (not shown) that may be connected to the metallic grid 30 would constitute the (−) terminal of the solar cell.

After fabrication, individual solar cells are typically assembled into solar cell strings or circuits by interconnecting them in series electrically, i.e. by connecting the (+) terminal of one cell to the (−) terminal of a neighboring cell. This way the total voltage of the solar cell circuit is increased. The solar cell circuit is then laminated into a protective package to form a photovoltaic module.

As shown in FIG. 1B the metallic grid 30 or the grid pattern is deposited on the illuminated side of the solar cell device and includes one or more busbars 32 and multiple fingers 34 to carry the current from various parts of the device to the busbars 32. Busbars 32 and fingers 30 generally comprise metals with low electrical resistivity such as silver or silver alloys, which can be ink-deposited or screen printed over the illuminated surfaces using silver-based inks or pastes.

Although the low electrical resistivity of such materials plays an important role in their choice, in operation, there is a trade off relationship between their size, i.e. height and width, and their electrical resistance, which critically depends on the cross sectional area of the fingers and the busbars. Since the fingers are spread over the illuminated surface, in order to reduce the shadowing effect caused by their presence on the illuminated surface, their width needs to be minimized while their height needs to be maximized to keep the cross sectional area high and therefore the resistance low. However, in ink deposition or screen printing approaches, when the width of the finger is reduced to minimize the shadowing loss, the height of the finger also gets reduced due to the nature of these processes and the nature of the inks and pastes used. Therefore, for narrow fingers the cross sectional area gets reduced and the resistance of the finger increases causing the overall efficiency of the solar cell to go down despite the fact that more light enters the device. It should be noted that resistivity and bulk resistivity mean the same in this application and they have the units of “ohm-cm”. Sheet resistance of a layer is defined as the resistivity of the material making up the layer divided by the thickness of the layer and has the units of “ohms per square”. The resistance of a conductive line, which has the units of “ohms” is equal to the resistivity of the material making up the line times the length divided by the cross sectional area of the line.

From the foregoing, there is a need in the thin film solar cell industry for improved grid structures and manufacturing methods that allows fabrication of narrow fingers with low resistance so that the conversion efficiency of the solar cells may be improved.

SUMMARY

The present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells.

In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in a adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece.

In a preferred aspect there is described a method of roll to roll manufacturing low electrical resistivity conductive grids having reduced shading effect for solar cells, comprising: providing a flexible continuous workpiece, the flexible continuous workpiece comprising a continuous flexible substrate, a bottom contact layer disposed atop the continuous flexible substrate, an absorber layer disposed atop the bottom contact layer, a transparent conductive layer disposed atop the absorber layer, and a first conductive film having a first resistivity disposed atop predetermined areas of a top surface of the transparent conductive layer and in electrical communication with the transparent conductive layer to form a raised grid pattern along a length of the flexible continuous workpiece, wherein the raised grid pattern includes a plurality of adjacent grids, with each grid having a predetermined grid width, a predetermined grid length, and a predetermined spacing between adjacent grids along the length direction of the flexible continuous workpiece, wherein a sheet resistance of the first conductive film is less than the sheet resistance of the transparent conductive layer, and wherein the top surface of the transparent conductive layer and the raised grid pattern disposed thereon form a front surface of the flexible continuous workpiece; applying an electrodeposition solution onto an effective plating region established on a portion of the front surface, including a part of the first conductive film, and onto an anode placed across from the portion of the front surface, the effective plating region having a length that is substantially the same as a width of the workpiece and a predetermined width that is at least longer than the predetermined spacing between adjacent grids; applying a voltage between the anode and the part of the first conductive film; selectively electrodepositing a conductive material from the electrodeposition solution onto the first conductive film and not the transparent conductive layer to form a second conductive film having a second resistivity atop the first conductive film, thereby forming the low electrical resistivity conductive grids having reduced shading effect, wherein the first resistivity is greater than the second resistivity; and moving the front surface, including the part of the first conductive film, through the effective plating region, during the steps of applying the electrodeposition solution, applying the voltage, and selectively electrodepositing.

These and other aspects and advantages are described further herein

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side schematic view of a solar cell of the prior art;

FIG. 1B is a top schematic view of the solar cell with a conductive grid over the top surface;

FIG. 2 is a perspective depiction of a reel to reel electroplating system processing a workpiece to form a raised conductive grid according to a preferred embodiment;

FIG. 3 is a schematic side view of a portion of a solar cell structure including a raised conductive grid formed on the transparent conductive layer;

FIG. 4 is a schematic view of a top portion of the workpiece; and

FIG. 5A is schematic side view of an electroplating apparatus of the electroplating system;

FIG. 5B is schematic frontal view of the electroplating apparatus; and

FIG. 5C is schematic top view of the electroplating apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described herein are methods and apparatus to form low electrical resistance grid patterns over illuminated side of photovoltaic cells or solar cells. In one embodiment, initially a conductive grid pattern is formed, preferably by a screen printing or ink deposition technique, over a transparent conductive layer of a solar cell structure. In the following step, a conductive material is selectively electroplated over the conductive grid pattern using the electroplating apparatus. The electroplated conductive material increases the height of the conductive grid pattern and reduces its electrical resistance. It should be noted that the resistivity of an electroplated conductor such as electroplated Cu or Ag is lower than the resistivity of screen printed or ink deposited conductors such as Ag pastes.

FIG. 2 shows a depiction of a roll to roll or reel to reel electrodeposition system 100 to selectively deposit a conductor onto a first conductive film 102 shaped as a plurality of grid patterns formed on a front surface 104A at a front side 101A of a workpiece 105, with only those components necessary of this description to be understood illustrated, and it being understood that the actual roll to roll or reel to reel electrodeposition system 100 will have additional components therein. The conductor material may include silver or silver alloy or another low electrical resistance material. Each grid pattern may form a top terminal of a solar cell after the electroplating step and after cutting individual solar cells out of the workpiece. The first conductive film includes a conductive metal such as silver or a silver alloy or compound which may be deposited by techniques such as screen printing and ink jet printing. During the process, the work piece 105 is advanced from a supply spool 106A in a process direction ‘P’, passed through a deposition unit 108 and wrapped around a receiving spool 106B. The conductor is electrodeposited on the first conductive film 102 as the workpiece 105 is passed through the deposition unit 108. Both the electrodeposited conductive material and the first conductive film 102 underneath, form a raised conductive film 110 having the shape of the grid patterns, which will be called final grid patterns hereinafter. During the process a back side 101B of the workpiece 105 of the workpiece is supported by various support means such as support plates or rollers.

FIG. 3 shows a detailed cross sectional view of an exemplary portion of the workpiece 105 (FIG. 2) after the electrodeposition process. As shown in FIG. 3, the conductor deposited by the electrodeposition process forms a second conductive film 103 on the first conductive film 102. Therefore, the raised conductive film 110 comprises the first conductive film 102 deposited on the front surface 104A of the workpiece 105 and the second conductive film 103 selectively deposited on the first conductive film 102. As mentioned above, each grid pattern on the front side 101A of the workpiece 105 forms a top terminal for the future solar cells. Accordingly, the layers under each grid pattern form the structural components of the future solar cells as well. In this respect, the front surface 104A includes the surface of a transparent conductive layer 112, such as a buffer-layer/TCO stack, formed on an absorber layer 114 which may be a Group IBIIIAVIA absorber layer such as a CIGS absorber layer. TCO stands for transparent conductive oxide such as a ZnO layer, an indium tin oxide (ITO) layer or a stack comprising both ZnO and ITO. An exemplary buffer layer may be a (Cd, Zn)S layer. The absorber layer 114 is formed on a base 115 including a flexible substrate 118 and contact layer 116 formed on the flexible substrate 118. A preferred flexible substrate material may be a metallic material such as stainless steel, aluminum (Al) or the like. An exemplary contact layer material may comprise molybdenum (Mo).

FIG. 4 shows a portion of the front side 101A of the workpiece 105 during an instant of the electroplating process. The portion of the front side 101A includes the grid patterns of the first conductive film 102 and a final grid pattern of the raised conductive film 110 located at both sides of an effective plating region 120 in which the workpiece 105 is advanced so that the grid patterns of first conductive film 102 are selectively electroplated with the conductor to form the grid patterns of raised conductive film 110 or the final grid patterns. As will be described below the effective plating region 120 is an area that an electrodeposition device (see FIGS. 5A-5C) in a preferred embodiment can deposit the conductor on the grid patterns of the first conductive film 102 as they moved through the effective area 120, thus forming the final grid patterns.

As shown in FIG. 4, each grid pattern of the first conductive layer 102 includes busbars 122 and fingers 124. After electrodepositing the conductor within the effective plating region 120, the busbars 122 become raised busbars 123 and the fingers 124 become raised fingers 125, both the raised busbars and fingers forming the final grid pattern. As will be appreciated, the raised busbars 123 and the raised fingers 125 comprise the first conductive film 102 and the second conductive film 103. It is critical that, in order to electrodeposit the conductor onto the first conductive film 102, the sheet resistance of the first conductive film 102 must be less than the sheet resistance of the front surface 104A which is the surface of the transparent conductive layer 112. The sheet resistance of the first conductive film 102 deposited in the form of a finger pattern on the transparent conductive layer 112 is less than about one tenth, preferably less than about one hundredth of the sheet resistance of the transparent conductive layer, which is typically in the range of 5-20 ohms per square.

The width ‘W’ of the effective plating region is greater than the distance ‘d’ between the grid patterns of the first conductive film 102. This way it is assured that a portion of the first conductive film 102 or a portion of the already plated grid pattern is always in the effective plating region 120. Since the resistances of the first and second conductive films 102 and 103 are much lower than that of the transparent conductive layer 112, the plating current preferentially passes through the fingers 124 and/or the raised fingers 125, depositing material there rather than on the transparent conductive layer. It should be noted that the bulk resistivity of the Ag-based material forming the first conductive film 102 is in the range of 10-30 micro-ohm-cm, whereas the resistivity of materials forming the transparent conductive layer 112 (FIG. 3) is in the range of 200-500 micro-ohm-cm. Furthermore the thickness of the first conductive film 102 is in the range of 1-10 microns, whereas the thickness of the transparent conductive layer 112 is typically in the range of 0.1-0.5 microns. As a result, the sheet resistance of the transparent conductive layer 112 is typically 100-5000 times larger than the sheet resistance of the first conductive layer. This differential facilitates the preferential plating on the first conductive layer 102 if there is, at all times, a section of the grid pattern within the effective plating region 120 and there is at least one electrical contact made to that grid pattern as will be further described. It should also be noted that the electroplated conductor or the second conductive film 103 typically has a very low resistivity in the range of 2-10 micro-ohm-cm, and therefore its thickness can be lower than the thickness of the first conductive film 102. For example, the thickness of the second conductive film 103 may be in the range of 1-5 microns.

FIGS. 5A, 5B and 5C show in side, top and front view, respectively, an electrodeposition apparatus 130 through which the workpiece 105 is advanced in the process direction ‘P’, during the electrodeposition process. A support member 131, such as a plate or a series of rollers, mechanically supports the workpiece portion that is being processed by the apparatus 130. As shown in FIG. 5A, the electrodeposition process applied by the apparatus 130 forms the raised fingers 125 from the fingers 124 by electrodepositing the conductive material onto the fingers 124, and thereby increasing its thickness and conductivity, while the workpiece 105 is advanced. The electrodeposition apparatus 130 is located in the deposition unit 108 of the electrodeposition system 100 shown in FIG. 2. As its components shown in FIGS. 5A and 5C, the electrodeposition apparatus 130 includes an electrodeposition cell 132, surface contacts 134 and a power supply 138. The electrodeposition cell 132 includes a substantially rectangular chamber 140 (see FIG. 5C) including long side walls 142A and 142B and short side walls 142C and 142D. The long side walls 142A and 142B extend along the width of the workpiece 105 and are separated by the distance ‘W’ which is also the width of effective plating region 120 shown in FIG. 5C and also in FIG. 4 in this embodiment. Adjacent the lower ends of the long side walls 142A and 142B, an entrance opening 149A and an exit opening 149B are located respectively. The short side walls 142C and 142D which are parallel to the edges of the workpiece 105 complete the rectangular chamber 140 which retains an electrodeposition electrolyte 146 and an electrode 148 or anode immersed into the electrolyte 146. FIG. 5B shows in front view the long side wall 142A, the entrance opening 149A and the position of the workpiece 105 entering through the entrance opening 149A of the electrodeposition cell 132 as the workpiece is advanced in the process direction ‘P’. As shown in FIGS. 5A and 5C, during the process the workpiece 105 enters the electrodeposition cell 132 through the entrance opening 149A and leaves the electrodeposition cell through the exit opening 149B while being supported by the support 131. In a preferred embodiment, a sufficient amount of the electrolyte 146 is maintained in the chamber 140 by being continuously or periodically filled from the top of the chamber 140 at an overall rate that accounts for the removal of the electrolyte 146 through the entrance and exit slits 147A and 149A, although it will be understood that other arrangements could be used to maintain the environment necessary for the electrodeposition to occur.

As the workpiece 105 is advanced through the electrodeposition cell 132, the electrodeposition electrolyte 146 flows towards the front side 101A of the workpiece 105, contacts it and flows out of both the entrance opening 149A and the exit opening 149B. The electrolyte 146 is pumped into the chamber 140 from an electrolyte supply tank (not shown) and the used electrolyte leaves the cell through the entrance opening 149A and the exit opening 149B. This used electrolyte may be flowed into a recycling tank (not shown) to filter and replenish it. The replenished electrolyte is then redirected into the electrodeposition cell 132 or the electrolyte supply tank (not shown). In this embodiment, the side walls 142A and 142B of the rectangular chamber 140 and the edges of workpiece as they pass through the plating chamber define the effective plating region 120.

The surface contacts 134 may be made of conductive rollers or brushes which negatively polarize the surface 104A and the first conductive film 102 which is shown as the finger 124 in FIG. 5. As shown in FIGS. 5A and 5C, there may be at least two surface contacts positioned at both sides of the cell 132 and they may extend along the width of the workpiece 105. If the surface contacts are made of conductive rollers, they roll on the surface as the workpiece travels. The anode electrode 148 and the surface contacts 134 are electrically connected to a positive and negative terminals of the power supply 130, respectively.

As can be seen in FIGS. 5A and 5C, the effective plating region 120 defined by the distance ‘W’ between the long side walls 142A, 142B and the edges of the workpiece within the electrodeposition cell 132 and thus the electrodeposition occurs in this region. As shown, the distance W is kept greater than the distance ‘d’ between the grids of the first conductive layer so as to leave at least a portion of the finger 124 or the raised finger 125 within the effective plating region. Since the sheet resistance of the finger 124 is lower than the sheet resistance of the surface 104A, the conductive material only deposits onto the fingers. Referring to FIG. 4 and FIGS. 5A and 5C, position of the surface contacts 134 is also predetermined depending on the length ‘L’ of the grid pattern so that at least one of the surface contacts 134 stays on the grid patterns. Further, the distance between the surface contacts should be less than or equal to the length of the fingers so that when a portion of a finger is in the effective plating region that particular finger is always contacted at least one surface contact outside the effective plating region.

Therefore, in one embodiment a finger plating or grid plating method comprises the steps of: i) providing a continuous flexible workpiece with two edges and a width, the workpiece comprising multiple solar cell structures on its front surface, each solar cell structure having a conductive grid pattern with fingers which are parallel to the two edges of the workpiece, ii) applying an electrodeposition solution onto an effective plating region on the front surface of the workpiece and onto an anode placed across from the front surface of the workpiece, the effective plating region having a length that is substantially the same as the width of the workpiece and a predetermined width that is larger than a distance between the grid patterns of adjacent solar cell structures, iii) applying a voltage between the anode and two contacts that touch the front surface of the workpiece while moving the workpiece and the effective plating region with respect to each other and in a direction that is substantially parallel to the fingers of the grid patterns thus causing electrodeposition of a conductive material from the electrodeposition solution onto the conductive grid patterns of solar cell structures, wherein the two contacts are provided on two sides of the effective plating region and a distance between the two contacts is less than or equal to the total length of each of the fingers of the grid patterns.

Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art. 

1. A method of roll to roll manufacturing low electrical resistivity conductive grids having reduced shading effect for solar cells, comprising: providing a flexible continuous workpiece, the flexible continuous workpiece comprising a continuous flexible substrate, a bottom contact layer disposed atop the continuous flexible substrate, an absorber layer disposed atop the bottom contact layer, a transparent conductive layer disposed atop the absorber layer, and a first conductive film having a first resistivity disposed atop predetermined areas of a top surface of the transparent conductive layer and in electrical communication with the transparent conductive layer to form a raised grid pattern along a length of the flexible continuous workpiece, wherein the raised grid pattern includes a plurality of adjacent grids, with each grid having a predetermined grid width, a predetermined grid length, and a predetermined spacing between adjacent grids along the length direction of the flexible continuous workpiece, wherein a sheet resistance of the first conductive film is less than the sheet resistance of the transparent conductive layer, and wherein the top surface of the transparent conductive layer and the raised grid pattern disposed thereon form a front surface of the flexible continuous workpiece; applying an electrodeposition solution onto an effective plating region established on a portion of the front surface, including a part of the first conductive film, and onto an anode placed across from the portion of the front surface, the effective plating region having a length that is substantially the same as a width of the workpiece and a predetermined width that is at least longer than the predetermined spacing between adjacent grids; applying a voltage between the anode and the part of the first conductive film; selectively electrodepositing a conductive material from the electrodeposition solution onto the first conductive film and not the transparent conductive layer to form a second conductive film having a second resistivity atop the first conductive film, thereby forming the low electrical resistivity conductive grids having reduced shading effect, wherein the first resistivity is greater than the second resistivity; and moving the front surface, including the part of the first conductive film, through the effective plating region, during the steps of applying the electrodeposition solution, applying the voltage, and selectively electrodepositing.
 2. The method of claim 1, wherein each grid includes fingers disposed parallel to the flexible continuous workpiece edges extending along the length of the flexible continuous workpiece.
 3. The method of claim 2, wherein in the step of moving, portions of the flexible continuous workpiece are continuously advanced into a front side of the effective plating region and through the effective plating region to form the second conductive film and then continuously advanced out of the effective plating region through a back side of the effective plating region after forming the second conductive film.
 4. The method of claim 3 wherein the step of selectively electrodepositing includes applying a first electrical contact adjacent the front side of the effective plating region and a second electrical contact adjacent the back side of the effective plating region, wherein a distance between the first and second contacts is less than or equal to the length of each of the fingers.
 5. The method of claim 4, wherein in the step of moving the portions of the flexible continuous workpiece are released from a supply roll of the flexible continuous workpiece and wound as a receiving roll when advanced out of the -effective plating region.
 6. The method of claim 4, wherein the first and the second contacts are conductive roll contacts rolling on the front surface as the flexible continuous workpiece is advanced.
 7. The method of claim 4, wherein the first and the second contacts are conductive brush contacts sweeping the front surface as the flexible continuous workpiece is advanced.
 8. The method of claim 1, wherein the first resistivity of the first conductive film is in the range of 10-30 micro ohm-cm, the second resistivity of the second conductor is in the range of 2-10 micro ohm-cm, and the resistivity of the transparent conductive layer is in the range of 200-500 micro ohm-cm.
 9. The method of claim 1, wherein the first conductive film includes a silver (Ag) based conductive material formed using one of a screen printing process and an ink jet printing process.
 10. The method of claim 1, wherein the second conductive film includes one of copper, silver, a copper alloy and a silver alloy.
 11. The method of claim 1, wherein the transparent conductive layer includes a stack including a transparent buffer layer deposited over the absorber layer and a transparent conductive oxide (TCO) layer deposited over the transparent buffer layer, and wherein the transparent buffer layer includes one of CdS and ZnS, and the TCO layer includes one of ZnO and indium tin oxide (ITO).
 12. The method of claim 1, wherein the absorber layer includes a group IBIIIAVIA compound semiconductor.
 13. The method of claim 1, wherein the substrate includes one of a stainless steel foil and an aluminum foil.
 14. The method of claim 1, wherein the bottom contact layer includes at least one of Mo, W, Ta, Ti, Cr and Ru materials.
 15. The method of claim 1, wherein the effective plating region is defined by an enclosure including a front wall and a back wall extending along the length of the effective plating region and two side walls extending along the width of the effective plating region.
 16. The method of claim 15, wherein the flexible continuous workpiece enters the effective plating region through an entrance opening the in the front wall and exits the effective plating region through an exit opening in the back wall.
 17. The method of claim 16, wherein the electrodeposition solution is delivered through a top opening of the enclosure and used electrodeposition solution flows out of the enclosure through at least one of the entrance and exit openings.
 18. The method of claim 5, wherein the effective plating region is defined by an enclosure including a front wall and a back wall extending along the length of the effective plating region and two side walls extending along the width of the effective plating region, wherein the front and the back walls forms the front and back sides of the effective plating region, respectively.
 19. The method of claim 18, wherein the electrodeposition solution is delivered through a top opening of the enclosure and used electrodeposition solution flows out of the enclosure through at least one of the entrance and exit openings.
 20. The method of claim 19, wherein the first and the second contacts are conductive roll contacts rolling on the front surface as the flexible continuous workpiece is advanced.
 21. The method of claim 19, wherein the first and the second contacts are conductive brush contacts sweeping the front surface as the flexible continuous workpiece is advanced.
 22. The method of claim 1, wherein the thickness of the first conductive film is in the range of 1-10 microns.
 23. The method of claim 1, wherein the thickness of the second conductive film is in the range of 1-5 microns.
 24. The method of claim 1, wherein the thickness of the transparent conductive layer is in the range of 0.1-0.5 microns. 